This invention relates to a ceramic-pyrophyllite-clay-silicate composition useful in the production of millboard and particularly useful in the manufacture of the carrying and/or lehr rolls utilized in high temperature glass manufacturing.
In glass making processes there typically occur continuous ribbon and/or discrete plates of glass, where annealing and hardening respectively occur, and along which it is necessary to continuously convey hot sheets or ribbons of glass. It is the function of the carrying or lehr rolls to accomplish this transfer of the hot sheets or ribbons of glass. Such rolls are typically fabricated by cutting discs from millboard and assembling and compressing the discs on a steel mandrel. Rolls which are so prepared are useful for supporting and moving glass from the molten state through an annealing oven or lehr to the finished hot glass plate.
Rolls fabricated in this manner are useful in transporting plate glass through tempering and/or hardening processes involving temperatures up to about 820.degree. C.
In the manufacture of conveying and/or lehr rolls employing ceramic-pyrophyllite-clay composition millboard, it is important that the millboard making up the lehr roll be capable of withstanding temperatures in excess of about 650.degree. C. for substantial periods of time. The conveying and/or lehr rolls must provide a heat resistant surface capable of conveying hot glass without damage to the glass surface. Furthermore, the conveying or lehr rolls must be capable of withstanding high temperatures and physical wear over extended periods of time.
Heretofore, the principal fiber used in lehr and conveying roll millboard compositions has been asbestos fiber. However, in response to adverse publicity about asbestos and the recent increase in the number of government regulations concerning asbestos, a large glass manufacturer decided to discontinue the use of asbestos-containing rolls. This decision and the expressed inclination of other glass manufacturers to discontinue the use of asbestos-containing rolls have underscored the need for a non-asbestos millboard composition.
Various insulating and/or refractory compositions including non-asbestos containing compositions have been previously disclosed. Thus, for example, U.S. Pat. No. 3,042,578 discloses an insulating product consisting of 60-70 percent perlite, 15-30 glass fiber, particularly asbestos and/or kraft fiber, and 10-25 percent sodium silicate. U.S. Pat. No. 3,658,564 discloses insulating structures comprising perlite, sodium silicate and inert fillers including clay, cellulosic materials and alumino-silicate fibers. U.S. Pat. No. 3,933,514 discloses an insulating composition which includes sodium silicate, perlite and fibrous materials such as ceramic fiber, cellulosic fiber and clay fiber. Finally, U.S. Pat. No. 4,090,881 discloses a high temperature refractory adhesive comprising mullite, ceramic fiber, colloidal silica, alumina and bentonite clay.